CA1334018C

CA1334018C - High corrosion resistant plated composite steel strip and method of producing same

Info

Publication number: CA1334018C
Application number: CA000586933A
Authority: CA
Inventors: Teruaki Izaki; Makoto Yoshida; Masami Osawa; Seijun Higuchi; Hisaaki Sato
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1987-12-29
Filing date: 1988-12-22
Publication date: 1995-01-17
Anticipated expiration: 2012-01-17
Also published as: DE3851425T2; AU601094B2; AU2751688A; EP0323756A1; DE3851425D1; US5082536A; KR890010288A; KR910007162B1; US4910095A; EP0323756B1

Abstract

An electroplated composite steel strip having a high corrosion resistance comprises a steel strip substrate and a corrosion resistant coating layer which comprises at least a base plating layer comprising a zinc-based metal matrix, a number of corrosion-preventing fine solid particles consisting essentially of core fine particles of, for example, chromate, phosphate or aluminum, molybdenum or titanium compounds, and encapsulated by very thin coating membranes consisting of, for example, SiO2 , Al2O3 , ZrO2 or TiO2 , and optionally a number of additional fine particles consisting essentially of, for example, SiO2 , TiO2 , Cr2O3 , Al2O3 , ZrO2 , SnO2 or Sb2O5.

Description

l- 133~01~

HIGH CORROSION RESISTANT PLATED COMPOSITE
STEEL STRIP AND METHOD OF PRODUCING SAME

BACRGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to a high corrosion resistant plated composite steel strip and a method of producing the same. More particularly, the present invention relates to a corrosion resistant plated composite steel strip having a corrosion-pre-venting zinc-based plating layer containing corrosion-preventing fine particles in the form of microcapsules having a very thin coating membrane, and a method of producing the same.

2. Description of the Related Art It is known that, in the winter in North America and Europe, the freezing (icing) of road surfaces is prevented by sprinkling rock salt powder or calcium chloride powder on the road surface, and that the above mentioned icing-preventing material causes a corrosion and rusting of the bodies of cars traveling on those roads.
Accordingly, there is a demand for a high corrosion resistant plated steel strip for car bodies which can be used under the above-mentioned circum-stances, without allowing the forming of red rust on the car bodies, over a long period.
There are two approaches for meeting the above-mentioned demand.
In countries, for example, the U.S.A.
and Canada, where the cost of electricity is relatively low, the corrosion resistance of the steel strip is promoted by forming a thick corrosion resistant coating layer on the steel strip. This is thick coating layer, however, causes the resultant coated steel strip to exhibit a reduced weldability, ~ - 2 ~ 1334018 paint adhesion, and plating properties.
In other countries, for example, Japan, where electricity is expensive and enhanced weldability, paint adhesion, and plating properties are required for the steel strip to be used for car bodies, a plated steel strip having a thin corrosion resistant electroplating layer has been developed.
The plated steel strip of the present invention belongs to the above-mentioned category of plated steel strips having a thin corrosion resistant electroplating layer.
In this type of conventional electroplated steel strip having a thin electroplating layer, a zinc alloy, for example, a zinc-iron, zinc-nickel of zinc-manganese alloy, is plated on a steel strip substrate, or zinc or a zinc-nickel alloy is electroplated on a steel strip substrate and a chromate treatment and an organic resinous paint are then applied to the electroplating layer. The zinc alloy-electroplated or zinc or zinc alloy-electroplated and painted steel strips have a thin coating layer at a weight of 20 - 30 g/m2. The conventional electroplated steel strips having the above-mentioned thin coating layer are not considered satisfactory for attaining the ob~ect of the domestic and foreign car manufacturers, i.e., that the car bodie~ should exhibit a resistsnce to corrosion to an extent such that rust does not form on the outer surface~ of the car bodies over a period of use of at least S years, and perforation from the outer and inner surfaces of the car bodies does not occur over a period of use of at least 10 years. In particular, a 10 year resistance to perforation is demanded.
Under the above-mentioned circumstances, investigations have been made into ways and means of obtaining a high corrosion resistant steel strip having a coating layer in which corrosion resistive fine solid particles are co-deposited with a plating metal matrix _ ~ 3 ~ 133~018 and are evenly dispersed within the plating metal matrix, i.e., a hiqh corrosion resistant plated composite steel strip.
The co-deposited, dispersed fine solid particles can impart various properties to the plating layer of the plated composite steel strip, and thus this co-deposition type plating method has been developed as a new functional plating method. Namely, this type of plating method has been recently disclosed in Japanese 1 0 Unexamined Patent Publication Nos. 60-96786 (published May 30, 1985), 60-211094, 60-211095 and 60-211096 (each published October 23, 1985).
Japanese Unexamined Patent Publication No.
60-96786 discloses a method of producing a plated composite steel strip in which fine solid particles of rust-resistant pigments, for example, PbCrO4 , SrCrO4 , 4 , CrO4 , Zn3(PO4)2 are co-deposited with a plating metal matrix, for example, Zn or a Zn-Ni alloy, to be evenly dispersed in the plating metal matrix.
This type of plated composite steel strip is considered to have an enhanced resistance to rust and perforation.
Nevertheless, according to the results of a tudy by the inventors of the present invention, the plated composite steel strip of Japanese Unexamined Patent Publication No.60-96786, in which the fine ~olid particles dispersed in the plating layer consist of rust-resistant pigments consisting of substantially water-insoluble chromates, for example, PbCrO4 SrCrO4 , ZnCrO4 or BaCrO4 , cannot realize the above-mentioned corrosion resistance level of no rust for at least 5 years and no perforation for at least 10 years. This will be explained in detail hereinsfter.
Generally, the rust resistant pigment fine particles of the substantially water-insoluble chromates dispersed in a zinc-plating liquid exhibit a surface potential of approximately zero, and accordingly, when a steel strip i~ placed as a cathode in the zinc-plating liquid and is electrolytically treated, zinc ions are ~ 4 ~ 1 33~018 selectively deposited on the steel strip surface but there i~ a resistance to the deposition of the rust resistant pigment fine particles into the zinc-plating layer, and therefore, it is very difficult to obtain a plated composite steel strip having an enhanced corrosion resistance.
Japanese Unexamined Patent Publication No. 60-211095 discloses a plated composite steel strip having a Zn-Ni alloy plating layer in which fine solid particles of metallic chromium, alumina (Al2O3) or silica (SiO2) are co-deposited with and dispersed in a Zn-Ni alloy matrix. According to the disclo~ure of this Japanese Publication 1095, the metallic chromium is obtained from chromium chloride (CrCl3); i.e., chromium chloride is dissolved in the plating liquid and releases chromium ions (Cr3 ), and when the steel strip is immersed and electrolytically plated as a cathode in the plating liquid, metallic chromium particles and chromium oxide (Cr2O3-nH20) particles are deposited into the plating layer to form a Zn-Ni alloy plating layer containing metallic chromium (Cr) and chromium oxide (Cr2O3-nH2O) particles.
When alumina or silica particles are further co-deposited into the Zn-Ni-Cr-Cr2O3.nH2O plating layer, the resultant plated composite steel strip exhibits an enhanced corrosion resistance compared with the plated composite steel having the Zn-Ni-Cr-Cr2O3.nH2O layer, but the degree of enhancement of the corrosion resistance is small, and the Al2O3 or SiO2 particle-con-taining, plated composite ~teel strip cannot realize aperforation resistance for at least 10 years.
Under the above-mentioned circumstances, it is desired by industry, especially the car industry, that a high corrosion resistant plated composite steel strip having a rust resistance for at least 5 years and a perforation resistance for at least 10 years, and a method of producing the same, be provided.

- - s -133~018 SU~ARY OF THE INVENTION
An ob~ect of the present invention is to provide a high corrosion resistant plated composite steel strip having an enhanced rust resistance for a period of at least 5 years and a perforation resistance for a period of at least 10 years, and a method of producing the same.
The above-mentioned ob~ect can be attained by the high corrosion resistant plated composite steel strip of 1~ the present invention which comprises:
(A) a substrate consisting of a steel strip; and (B) at least one corrosion resistant coating layer formed on at least one surface of the steel strip substrate and comprising a base plating layer which comprises (a) a matrix consisting of a member selected from the gro~p consisting of zinc and zinc alloys; and (b) a plurality of corrosion-preventing fine solid particles dispensed in the matrix and consisting essentially of fine core solid particles encapsulated by organic or inorganic membranes.
The fine core inorganic solid particle preferably comprises at least one member selected from the group consisting of chromates, aluminum compounds, phosphates, molybdenum compounds and titanium compounds.
The high corrosion re~i~tant plated composite steel strip mentioned above i8 produced by the method of the present invention which comprises;
coating at least one surface of a substrate consisting of a descaled steel strip by at least first electroplating the substrate surface with a first electroplating liquid containing (a) matrix-forming metal ions selected from the group consisting of zinc ions and mixtures of ions of zinc and at least one metal other than zinc to be alloyed with zinc, (b) a plurality of corrosion-preventing fine solid particles dispersed in the electroplating liquid and consisting of fine core _ - 6 - 13340~8 solid particles encapsulated by orgsnic or inorganic coating membranes, and (c) a co-deposition-promoting agent for promoting the co-deposition of the corrosion-preventing fine particles together with the matrix-forming method, to form a base plating layer on the substrate surface.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows the corrosion resistances of an embodiment of the high corrosion resistant plated composite steel strip of the present invention, two comparative conventional plated composite steel strips, and a comparative conventional zinc-galvanized steel strip;
Fig. 2 shows the relationship between the pH of the plating liquids and the amounts of substantially water-insoluble chromate particles deposited from the plating liquids;
Fig. 3 shows a relationship between a concentration of Cr6 ions in a plating liquid and an amount of substantially water-insoluble chromate particles deposited from the plating liquid;
Fig. 4 shows a relationship between an oxidation-reduction reaction time of metallic zinc grains with Cr6+ ions in a plating liquid and a concentration of Cr6+ ions in the plating liquid; and, Figs. SA, SB, SC, and SD, respectively, are explanatory cross-sectional views of an embodiment of the plated composite steel strip of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the high corrosion resistant plated composite steel strip of the present invention, at least one surface of a steel strip substrate is coated with a corrosion resistant coating layer comprising at least a base electroplating layer.
The base electroplsting layer comprises ~ plating matrix consisting of zinc or a zinc alloy and a plurality - _ 7 _ 1 33 40 1~

of corrosion-preventing fine solid particles evenly dispersed in the matrix. The corrosion-preventing fine particles consist essentially of fine core solid particles encapsulated by organic or inorganic ~ membranes and are in the form of microcap6ules.
In the plated composite steel strip of the present invention, preferably the base plating layer is formed on the steel strip substrate surface in a total amount of from 5 to 50 g/m2, more preferably from 10 to 40 gJm .
In the base electroplating layer of the present invention, the matrix thereof consists of zinc or a zinc alloy. The zinc alloy consists of zinc and at least one additional metal member to be alloyed with zinc. The additional metal member is preferably selected from the group consisting of Fe, Co, Mn, Cr, Sn, Sb, Pb, Ni, and Mo. The content of the additional metal member in the zinc alloy is not limited to a specific level.
The base plating layer optionally contains a plurality of additional fine or colloided particles comprising at least one member selected from the group consisting of 2 2 ~ Cr23 ~ A1203 ~ ZrO2 , SnO2 and Sb O
The corrosion-preventing fine solid particles in the form of microcapsules consist essentially of fine core solid particles, for example, particles of water-soluble or water-low soluble chromates; aluminum compounds, phosphates, molybdenum compound~, and titanium compounds, and organic or inorganic coating membranes formed around the core particles.
The water-soluble chromates include, for example, CrO3 , Na2CrO4 , g2CrO4 , K20.4ZnO.4CrO3. The water-low soluble chromates include, for example, PbCrO4 , BaCrO4 , SrCrO4 and ZnCrO4. The aluminum compounds include, for example, Zn-Al alloys and A1203-2SiO2.2H20.
The phosphates include, for example, Zn3(P04)2.2H20.
The molybdenum compounds include, for example, ZnO.ZnMoO4 , CaMoO4-ZnOMoO4 and PbCrO4-PbMoO4.PbS04.

_ - 8 ~ 133~18 The titanium compounds include, for example, Tio2-Nio-sb2o3.
The core fine particles may consist of an organic substance, for example, fluorine-containing polymer S resin~ or polypropylene re~ins.
The coating membrane formed around the core particle preferably has a thickness of 1.0 ~m or less and comprises at least one member selected from inorganic materials, for example, SiO2 , TiO2 , A1203 and ZrO2 and organic materials, for example, ethyl cellulose, amino resins, polyvinylidene chloride resins, polyethylene resins, and polystyrene resins.
The corrosion-preventing fine solid particles in the form of microcapsules have the following effects and advantages.
(1) The conventional corro~ion-resistant fine particles, for example, chromate and pho~phate particles, exhibit a ~urface potential of substantially zero or a very small value in an electroplating liquid.
Accordingly, in the electroplating process in which an electrophoretic property of particle is utilized, the co-deposition property of the conventionsl corrosion-resistant fine particles is unsati~factory. The SiO2 , TiO2 , A1203 , or ZrO2 exhibit a satisfactory surface potential in the electroplating liquid, even when in the form of a very thin membrane. Therefore, the fine ~olid particle of the present invention consisting essentially of a core solid particle consisting of a corrosion-resistant but non-electrophoretic material, for example, chromate, phosphate, aluminum compound, molybdenum compounds or titanium compound and a membrane consisting of an electrophoretic material, for example, SiO2 , TiO2 , A1203 , ZrO2 , exhibit a satisfactory electrophoretic and co-deposition property.
(2) The corrosion-preventing core particles, for example, a chromate or phosphate have a relatively high solubility in the electroplating liquid and the coating membranes have ~ substantially no or a very low solubility in the electroplating liquid.
For example, a water-low soluble chromate particle is dissolved in a small amount in the electroplating liquid and generates Cr6 ions. When the concentration of Cr6 ions in the electroplating liquid reaches a predetermined level or more, it causes the amount of the deposited particles to be decreased, and the resultant plating layer on a ~ubstrate exhibits an undesirable black powder-like appearance and a low adhe~ion to the ~ubstrate.
Accordingly, when the corrosion resistant core particles are coated with the insoluble membranes, the resultant microcapsulated particles exhibit a satisfactory resistance to dissolution in the electroplating liquid, and the electroplating liquid i8 maintained in a satisfactory stable condition over a long period and produces a plated composite steel strip having a high quality.

(3) The microcapsulated particles of the present invention dispersed in the base plating layer enhances the corrosion resistance of the plated composite steel strip over the conventional plated composite steel strip containing non-microcapsulated corrosion-resistant particles. Because the corrosion-preventing activity of the core particles is promoted by the coating membrane~, for example, SiO2 , TiO2 , A1203 or ZrO2 membranes, which have a high corrosion-resistance.
Referring to Fig. 1 which shows decreases in thickness of four different plated composite steel strips by a corrosion test, sample No. 1 is a plated composite steel strip which was produced in accordance with the method disclo~ed in Japanese Unexamined Patent Publication (Rokoku) No. 60-96,786 and had 23 g/m2 of an electroplating layer consisting of 8 zinc matrix and 0.3% by weight of BaCrO4 particles dispersed in the matrix.

-lo- 1334~18 Sample No. 2 is a plated composite steel strip which was produced in accordance with the-method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-211,095 and had 20 g/m2 of an electroplating layer consisting of a matrix consisting of zinc-nickel alloy containing 1% by weight of Ni and particles consisting of 1% by weight of metallic chromium (Cr) and chromium oxide particles and 1% by weight of A12O3 particles dispersed in the matrix.
Sample No. 3 is a plated composite steel strip of the present invention having 21 g/m2 of an electroplating layer consisting of a matrix consisting of a zinc-cobalt alloy containing 10% by weight of Co and 4.0% by weight of corrosion-preventing fine solid particles consisting of BaCrO4 core particles and SiO2 coating membranes and 1% by weight additional TiO2 particles.
Sample No. 4 is a zinc-galvanized steel strip which has 90 g/m2 of a thick zinc-galvanizing layer and is believed to exhibit a high perforation resistance over a long period of 10 years or more.
The corrosion test was carried out in such a manner that a corro~ion treatment cycle comprising the successive steps of a salt water-spraying procedure at a temperature of 35C for 6 hours, a drying procedure at a temperature of 70C at a relative humidity of 60%RH for 4 hours, a wetting procedure at a temperature of 49C at a relative humidity of more than 95~RH for 4 hours, and a freezing procedure at a temperature of -20C for 4 3 n hours, was repeatedly applied 50 times to each sample.
In Fig. 1, the perforation resistances of Sample No. 1, the plated zinc layer of which contained BaCrO4 particles, and Sample No. 2, the plated zinc-nickel alloy layer of which contained metallic chromium and chromium oxide particles and A12O3 particles, are poorer than that of Sample No. 4 having a thick (90 g/m2) galvanized zinc layer. Also, Fig. 1 shows - - - 133~018 that the perforation resistance of Sample No. 1, the plated zinc layer of which contain~ only a substantially water insoluble chromate (BaCrO4) particles in a small amount of 0.3% by weight, is unsatisfactory. That is, by the method of Japanese Unexamined Patent Publication (Kokoku) No. 60-96786, it i8 difficult to deposit a large amount of the rust-resistant pigment consisting of substantially water-insoluble chromate particles from the electroplating liquid into the zinc plating layer, because the chromate particles in the plating liquid have a surface potential of approximately zero.
Further, Fig. 1 shows that Sample No. 3, i.e., the plated composite steel strip of the present invention, exhibited a higher perforation resistance than that of Sample No. 4.
Namely, in the plated composite steel strip of the present invention, the microcapsule-like corrosion-preventing fine particles promote the perforation resistance-enhancing effect of the substantially water-insoluble chromate particles in the base electroplatinglayer.
The conventional corrosion re~istant particles dispersed in the base plating layer promote the corrosion resistance of the plating layer in the following manner. For example, when low water-soluble chromate particles are co-deposited together with a matrix-forming metal on a steel strip substrate to form a plating layer, and the resultant plated composite steel strip is placed in a corrosional circumstance, the chromate particles are decomposed with the development of the corrosion and generate Cr6+ ions. The Cr6 ions react with the metal in the plating layer to form corrosion resistant chromium compounds and chromium oxides and chromium hydroxide. This phenomenon is effective for providing a corrosion resistant layer in the plating layer and for enhancing the corrosion re~istance of the plating layer.

- - 12 - 133~018 When the chromium compound layer in the platinq layer i~ decomposed, a new corrosion resistant chromium compound layer is formed in the plating layer, because a number of chromate particles are evenly distributed in the plating layer.
The re-formation of the corrosion-resistant chromium compound layer is repeated.
When the microcapsule-like particles of the prevent invention are used, the corrosion resistant plating layer exhibits a promoted corrosion resistance in the following mechanism.
For example, microcapsule-like particles of the present invention comprising core particles consisting of low water-insoluble chromate and coating membranes consisting of SiO2 , a portion of the chromate is very slowly dissolved through the thin coating membranes, because practically, the coating membranes do not completely seal the core particles.
The generating rate of Cr6+ ion~ in the plating layer of the present invention is significantly smaller than that of the conventional plating layer in which the chromate particlec are not encapsulated, and thus the corrosion resistance of the plating layer can be maintained at a satisfactory level over a longer period than the conventional plating layer.
According to the inventor's study, the Cr6+ ion-forming rate in the plating layer of the present invention i8 about 1/3 to 1/10 times that in the conventional plating layer.
That is, the plated composite ~teel strip of the present invention hAs a long term corrosion resistance and can withstand a corrosion test over a period of 1 to 3 months, and can meet the demand of a 10 year resistance to perforation for car bodies.
The other types of core particles, for example, phosphate particles which generate P043 ions and molybdenum compound particles which generate MoO42 ~ - 13 ~ 133~018 ions, can exhibit the corrosion-preventing effect in the same mechanism as that of the chromate particles.
In the present invention, the corrosion resistant fine particles in the form of microcapsules are preferably contained in a total amount of 0.1% to 30%, more preferably 0.1% to 20% by weight, based on the weight of the base coating layer.
When the content of the corrosion-preventing fine particles is less than 0.1%, the resultant base plating layer sometimes exhibits an unsatisfactory corrosion resistance.
When the content of the corrosion-preventing fine particles is more than 30% by weight, the resultant base plating layer sometimes exhibits an unsatisfactory bonding property to the steel strip substrate.
The additional fine or colloidal particles to be dispersed together with the corrosion-preventing fine particles in the form of microcapsules, for example, 2 ' Ti2 ' Cr23 ~ A1203 , ZrO2 , SnO2 , and Sb205 , promote the corrosion resistance of the base plating layer as follows.
The additional fine or colloidal particles exhibit a lower corrosion-re~ist~nt property than that of the corrosion-preventing fine particles, but in the base plating layer, the additional fine or colloidal particles are distributed, between the corrosion-preventing fine particles, and thus can restrict the corrosion of the portion of base plating layer around the additional particles. Namely, the additional particles exhibit a b~rrier effect against corrosional action.
In the base plating layer of the present invention, the additional fine or colloidal particles are preferably in a content of from 0.1% to 30%, more preferably from 0.1% to 20%, based on the total weight of the base electroplating layer.
When the content of additional particles is less - 14 - 133~018 than 0.1% by weight, the improvement in the corrosion resistance of the base plating layer due to the additional particles i8 sometimes unsatisfactory. When the content of the additional particles is more than 30%
by weight, the resultant base plating layer sometimes exhibits a poor bonding property to the steel strip substrate.
Preferably, in general the total content of the corrosion-preventing fine particles and the additional particles does not exceed 30% based on the weight of the base plating layer.
In an embodiment of the composite steel strip of the present invention, the corrosion resistant coating layer has an additional thin electroplating layer formed on the base plating layer. The additional electro-plating layer preferably comprises at least one member selected from the group consisting of Zn, Fe, Co, Ni, Mn and Cr, and preferably has an amount of 1 to 5 g/m2.
In another embodiment of the composite steel strip of the present invention, the corrosion resistant coating layer has a surface coating layer formed on the base plating layer. The surface coating layer may have a single layer structure comprising a member selected from organic resinous materials and mixtures of at least one of the organic resinous materials and chromium ions.
The organic resinous materials include, for example, epoxy resins, epoxy-phenol resins and water-soluble type and emulsion type acrylic resins.
Alternatively, the surface coating layer has a double layer structure consisting essentially of an under layer formed by applying a chromate treatment to the base plating layer surface and an upper layer formed on the under layer and comprising an organic resinous material as mentioned above.
In still another embodiment of the composite steel strip of the present invention, the above-mentioned surface coating layer is formed on the above-mentioned lS- 1334018 additional thin electroplating layer on the base plating layer .
The additional electroplating layer and the surface coating layer will be explained in detail hereinafter.

5 - In the method of the present invention, at least one surface of a substrate consisting of a descaled steel strip i8 coated by at least first electroplating the substrate surface in a first electroplating liquid.
The surface of the steel strip to be first electroplated is cleaned by an ordinary surface-cleaning treatment, before the first electroplating step.
The first electroplating liquid contains (a) matrix-forming metal ions selected from zinc ions or a mixture of zinc ions and at least one other metal ion than zinc ions to be alloyed with zinc, (b) a number of the above-mentioned corrosion-preventing fine solid particles in the form of microcapsules, dispersed in the first electroplating liquid and (c) a co-deposition-promoting agent for promoting the co-deposition of the corrosion-preventing particles together with the matrix-forming metal, to provide a base electroplating layer on the substrate surface.
The first electroplating liquid optionally contains at least one type of additional fine or colloidal particles consisting of a member selected from the group consisting of SiO2 , TiO2 , Cr203 , A1203 , ZrO2 , SnO2 , and Sb205.
The co-deposition-promoting agent is used to promote the co-deposition of the corrosion-preventing particles, and optionally the additional particles, together with the matrix-forming metal, from the first electroplating liquid into the base electroplating layer. The co-deposition-promoting agent preferably comprises at least one member selected from the group consisting of Ni2 ions, Fe2 ions, Co2 ions, Cr3 ions, TiO2 colloid, A1203 colloid, SiO2 colloid, ZrO2 colloid, SnO2 colloid, and Sb205 colloid.

- 16 ~ 133 4018 The role of the above-mentioned ions or colloids as the co-deposition-promoting agent will be explained below.
As stated above, the surface potential of the corrosion-preventing particles in the electroplating liquid can be controlled by the thin coating membranes.
When the corrosion-preventing particles have thin SiO2 coating membranes, the resultant microcapsule-like particles have a negative surface potential.
In an electroplating process in which a steel strip serves as a cathode, it is difficult to deposit the microcapsules-like particles having the thin SiO2 coating membranes into the plating layer on the steel strip substrate. Accordingly, the deposition of the microcapsules-like particles into the plating layer must be promoted by using the co-deposition-promoting agent.
Where Ni2 ions are used as the co-deposition-promoting agent, the Ni2+ ions are absorbed on the surface of the SiO2 coating membrane surfaces of the microcapsule-like particles so that the surfaces of the microcapsule-like particles have a positive potential.
The microcapsule-like particles having the positive surface potential can be readily drawn to and deposited into the plating layer on the cathode (steel strip).
The Co2 , Fe2 and Cr3+ ions in the electroplating layer exhibit the same co-deposition-promoting effect as that of the Ni2 ions. The metal ions Ni2 , Co2 , Fe2 and Cr3 , are also deposited to form a zinc alloy matrix which is effective for enhancing the corrosion resistance of the fir~t electroplating layer.
2 ' 2 1 A123 ~ Zr2 ~ SnO2 and Sb 0 colloids added to the electroplating liquid serve as a co-deposition-promoting sgent in the same manner as that of the Ni2 ions, etc.
When added to the electroplating liquid, the colloid particles exhibit a positive or negative potential and are absorbed on the surfaces of the ~ - 17 _ 1334018 corrosion-preventing microcapsule-like fine particles.
For example, at a pH of l to 2.5, A12O3 , ZrO2 , SnO2 , and TiO2 colloid particles exhibit a positive potential, and SiO2 and Sb2O5 colloid particles exhibit a negative potential. Accordingly, the nature and intensity of the potential of the fine particles in the electroplating liquid can be ad~usted to a desired level by controlling the type and amount of the colloid particles to be added to the electroplating liquid, in consideration of the type of the electroplating method.
That i8, the composition of the co-deposition-promoting agent should be determined in view of the composition of the corrosion-preventing microcapsule-like particles, especially the type and nature of the thin coating membrane.
The co-deposition of the corrosion-preventing particles can be promoted by using another type of co-deposition-promoting agent which is very effective for the accelerated co-deposition of the corrosion-preventing particles and for stabilizing the electro-plating step for the base plating layer.
The co-deposition-promoting agent comprises at least one member selected from the group consisting of amine compounds having a cationic polar structure of the formula (1):

Rl - ~N''' (l) ammonium compounds having a cationic polar structure of the formula (2): 2 R

R1 _ ~ / R3 (2) \ R4 wherein R , R , R3 , and R4 represent, respec-tively and independently from each other, a member selected from the group consisting of a hydrogen atom, 133~018 and alkyl and aryl radicals, and polymers having at least one type of the cationir polar radical.
The amine compounds, ammonium compounds and the cationic polymers are selected, for example, ethylene imine >

and ethylene imine-containing polymers, diallylamine CH2 = CHCH
CH2 = CHCH2 diallylamine-containing polymers, polyaminesulfons which are copolymers of diallylamine and S02 , trimethyl-ammouium chlorides [R - N~ - CH3.Cl ], diallyldimethylammonium chloride CH2 = CHCH2 \ ~ ~C 3 Cl~]
CH2 = CHCH2 / \ CH3 and alkyl betaines Rl [R - N~ - CH2COOH]

The base plating layer of the present invention has a satisfactory rust-resistance and corrosional perfora-tion resistance, but it was found that, when some typesof the plated composite steel stripq are sub~ected to a chemical conversion treatment as a treatment prior to a point coating step, the base plating layer tends to hinder the growth of chemical conversion membrane crystals. That is, the chemical conversion membranes are formed only locally and the crystals in the membrane are coarse, and therefore, the chemical conversion lg 133~018 membrane exhibits a poor adhesion to the paint coating.
This disadvantage i8 serious when the base plating layer contains chromium-containing particles.
Accordingly, where a paint coating is required, for example, on a steel strip to be used for forming outer surfaces of the car bodies, preferably the base electroplating layer is coated with a thin additional electroplating layer, preferably in a weight of 1 to 5 g/m2. The additional electroplating layer preferably comprises at least one type of metal selected from the group consisting of Zn, Fe, Co, Ni, Mn, and Cr.
The base plating layer in the plated composite steel strip of the present invention may be coated with a surface coating layer having a coating structure selected from the group consisting of simple coating layers comprising an organic resinous material, and optionally, chromium ions evenly mixed in the paint, and composite coating layers each consisting of an under layer formed by applying a chromate treatment to the base electroplating layer surface and an upper layer formed on the under layer and comprising an organic resinous material. The surface coating layer effec-tively enhances the firm adhesion of the paint to the plated composite steel strip.
The above-mentioned surface coating layer may be further formed on the additional electroplating layer formed on the base electroplating layer.
In the method disclosed in Japanese Unexamined Patent Publication (Rokai) No. 60-96786, the first electroplating operation i~ carried out with a first electroplating liquid having a pH of 3.5 or more. Where the steel strip serves as a cathode and the electro-plating liquid has a pH of 3.5 or more, the pH at the interface between the cathode and the electroplating liquid is easily increased to a level of pH at which a membrane of Zn(OH2) is formed, the Zn(OH)2 membrane hinders the deposition of metal ions and the rust-- 20 - 133~018 resistant pigment particles having a larger size than that of the metal ions onto the cathode surface through the Zn(OH)2 membrane. That is, the formation of the electrocoating layer containing the corrosion-resistant dispersoid particles is obstructed by the Zn(OH)2 membrane formed on the cathode surface. Therefore, the resultant plating layer has an unstable composition, contains a very small amount of the corrosion resistant dispersoid particles, and thus exhibits an unsatis-factory corrosion resistance.
Referring to Fig. 2, which shows a relationship between the pH of the electroplating liquid and the-amount of low water-soluble chromate fine particles deposited from the electroplating liquid, it is clear that, at a pH of 3.5 or more, the amount of the deposited chromate fine particles becomes very small.
Al~o, it should be noted that a portion of the chromate particles is dissolved in the electroplating liquid to generate Cr6 ions. If the electroplating operation is carried out in an electroplating liquid containing a large amount of Cr6+ ions, the resultant electroplating,layer is formed by a black colored powder and exhibits a very poor adhesion to the steel strip substrate. Where the content of Cr6+ ions in the electroplating liquid is in the range of from 0.1 to 0.25 g/l, the black colored deposit is not formed in the resultsnt electroplating layer. However, the electro-plating layer contains a very small amount of the low water-soluble chromate fine particles deposited therein.
Figure 2 suggests that, in the range of a Cr6+ ion content of from 0.1 to 0.25 g/l in the electroplating liquid, an increase in the content of Cr6 ions results in remarkable decrease in the amount of the low water-soluble chromate fine particles deposited.
Also, referring to Fig. 3 showing a relationship between the content of Cr6 ions in an electroplating liquid and the amount of low water-soluble chromate fine particles deposited from the electroplating liquid, it is clear that the increase in the content of Cr6 result in a remarkable decrease in the amount of the deposited chromate fine particles, and at a Cr6 ion content of 0.3 g/l or more, practical electroplating becomes impossible.
In the method of Japanese Unexamined Patent Publi-cation (Kokai) No. 60-96786, an attempt i8 made to resolve the Cr6 ion problem in the following manner.
That is, where an electroplating liquid contains BaCrO4 fine particles as substantially water-insoluble chromate fine particles, a portion of the BaCrO4 is dissolved in the electroplating liquid and is dis-sociated by the following reaction.
BaCrO4~ ~ Ba + CrO4 (Cr The reaction in the~ direction causes the BaCrO4 to be dissolved in the electroplating liquid. To restrict the dissolution reaction, the ionic dis60ciation of the BrCrO4 should be prevented by, for example, adding Ba2+
ions. The addition of Cr6+ ions should be avoided, because the increase in the Cr6+ ion content in the electroplating liquid results in a decrease in the plating utility of the electroplating liquid.
To add Ba2+ ions, BaC12 , which has a relatively large solubility in water, is preferably added to the electroplating liquid. In the method of Japanese Unexamined Patent Publication No. 60-96786, the electroplating liquid contains chlorides including BaC12. However, when a non-soluble electrode is used as an anode in a chloride-containing electroplating liquid, chlorine gas is generated from the electroplating liquid. Therefore, a soluble electrode must be used as an anode in the chloride-containing electroplating liquid.
However, in most of the recent electroplating apparatuses, the electrode is a fixed type, and thus is a non-soluble electrode, because generally, in most recent electroplating methods, a horizontal, high flow speed type electroplating cell is used, the distance between the steel strip and electrode is made short to increase the current density to be applied to the electroplating process, and the plated steel strip is produced at a very high efficiency which corresponds to several times that obtained in a conventional electro-plating process.
The method of the present invention is very useful for electroplating a steel strip substrate in a horizontal, high flow speed type electroplating apparatuses at a high current density and at a high efficiency. In this type of electroplating process, when a non-soluble electrode i~ used, the electroplating liquid is preferably a sulfate type plating bath.
In the sulfate type plating bath, the generation of Cr6 ions cannot be prevented by adding Ba2 ions to the bath, because the added Ba2 ions are converted to BaS04 which is insoluble in water and deposits from the bath.
Accordingly, where the sulfate type plating liquid is used as a first electroplating bath for the method of the present invention, it is preferable to convert the dissolved Cr6+ ions to Cr3 ions by adding grains or a plate of a metal, for example, metallic zinc or iron, or a reducing agent, for example, sodium sulfite, in a necessary amount for reducing the dissolved Cr6 ions to Cr3+ in the first electroplating liquid. In this manner, an oxidation-reduction reaction is utilized.
Figure 4 shows a relationship between the reaction time (minute) of metallic zinc grains added in an amount of 20 kg/m3 in an electroplating liquid and the concen-tration (g/l) of Cr6+ ions dissolved in the electro-plating liquid. In view of Fig. 4, it is clear that, after the metallic zinc grains are added to the electroplating liquid, the Cr6+ ions are reduced to Cr3 ions by the reduction reaction of the zinc grains, and thus the concentration of the Cr6+ ions decreases with the lapse of the reaction time.
That is, it was found that a high corrosion resistant plated composite steel strip, in which a stable dispersion of the corrosion-resistant solid particles in a satisfactory amount in a base plating layer is ensured, can be easily produced by the method of the present invention in which, preferably, the pH of the first electroplating li~uid in controlled to a level of 3.5 or less, more preferably from 1 to 2.5, and the concentration of the dissolved Cr6 ions is restricted to a level of 0.1 g/l or less, more preferably 0.05 g/l or less, by adding metal grains or plate or a reducing agent to the first electroplating liquid, at a wide range of current density from a low level to a high level.
The resultant high corrosion resistant plated composite steel strip of the present invention exhibits an excellent metal plating and adhesion, weldability, and painting properties.
Referring to Fig. 5A, a plated composite steel plate is composed of a steel strip substrate 1 descaled by a ordinary surface cleaning treatment and a base plating layer 2, which consists of a metal matrix 2a consisting of zinc or a zinc alloy, for example, an alloy of zinc with at least one member selected from Fe, Co, Mn, Cr, Sn, Sb, Pb, Ni and Mo, and a number of corrosion-preventing microcapsule-like fine particles 3 of the present invention and additional fine or colloidal particles 4 consisting of a member selected 2 ' 2 ~ Cr23 ~ A1203 ~ ZrO2 , SnO2 and Sb205 .
Referring to Fig. 5B, a base plating layer 2 formed on a steel strip substrste 1 is coated by a thin additional electroplating layer 5, which comprises at least one member selected from Zn, Fe, Co, Ni, Mn and Cr. Prefersbly, the additional electroplating layer 5 is in sn amount of 1 to 5 g/m2. In Fig. 5C, a base electroplating layer 2 is coated with a coating layer 6.
The coating layer 6 may be a single coating layer structure made of an organic resinous material, which optionally contains chromium ions evenly mixed in the resinous material, or a double coating layer structure consisting of an under layer formed by applying a chromate treatment to the base plating layer surface and an upper layer formed on the under layer and comprising an organic resinous material as mentioned above.
As shown in Fig. 5D, the same coating layer 6 as mentioned above is formed on the additional electroplating layer 5 formed on the base electroplating layer 2.
The coating layer 6 is preferably formed when the base or additional electroplating layer contains chromium. When a chromium-containing compound, for example, the low water-soluble chromate, or metallic chromium is contained in an electroplating layer, and a chemical conversion treatment is applied as a pre-paint coating step to the surface of the electroplating layer, it is known that the resultant chemical conversion membrane contains coarse crystals. The coarse crystals cause the chemical conversion membrane to exhibit a poor paint coating property. Therefore, preferably a surface layer to be chemical conversion-treated is free from chromium compound or metallic chromium, The organic resinous material usable for the surface coating layer may be selected from epoxy resins, epoxy-phenol resin~, and water-soluble polyacrylic resin emulsion type resins.
The organic resinous material may be coated by any conventional coating method, for example a roll-coating method, electrostatic spraying method, and curtain flow method. From the aspect of ensuring the weldability and processability of the resultant plated composite steel strip, the thickness of the organic resinous material layer is preferably 2 ~m or less.

- 25 ~ 1 33 4018 In the surface coating layer, the organic resinous material layer is al~o effective for preventing the undesirable dissolution of chromium from the chromate-treated under layer, which is very effective for enhancing the corrosion resistance of the plated composite steel strip. The dissolution of chromium sometimes occurs when the plated composite steel strip having the chromate treatment layer is sub~ected to a degreasing procedure or chemical conversion procedure, and can be prevented by coating the chromium compound-containing layer with the resinous material layer, which optionally contains chromium ions.
Recently, a method of applying a new surface coating layer having a thickness of about 2 ~m and containing SiO2 particles, etc, to the electroplating layer has been developed. This surface coating layer consisting of an organic resinous material and the SiO2 particles can exhibit a high corrosion resistance without the chromate treatment or using chromium ions.
The present invention will be further explained by way of specific examples which, however, are representa-tive and do not restrict the scope of the present invention in any way.
ExamPles 1 to 38 and ComParative Exam~les 1 to 7 In each of the examples and comparative examples, a cold-rolled steel strip having a thickness of 0.8 mm, a length of 200 mm, and a width of 100 mm was degreased with an alkali aqueous solution, pickled with a 10%
sulfuric acid aqueous solution, and washed with water.
The descaled steel strip was sub~ected to a first electroplating procedure wherein the steel strip served as a cathode, a first electroplating liquid containing necessary metal ions, corrosion-preventing fine particles, additional fine or colloidal particles and a co-deposition-promoting agent, as shown in Table 1, was stirred and circulated through an electroplating vessel and a circulating pump, while controlling the amountQ of - - 26 _ 1 3 34~1~

the above-mentioned components to a predetermined level, and while maintaining the pH of the first electroplating liquid at a level of 2, and the electroplating operation was carried out at a temperature of about 50C at a current density of 40 A/dm2 for about 22 seconds to provide base electroplating layers in a targeted weight of 22 g/m2 formed on both surfaces of the steel strip.
For example, in each of Examples 22 to 25 in which the resultant base electroplating layer was composed of a matrix consisting of a zinc (90%) - cobalt (10%) alloy and corrosion-preventing fine particles consisting of 4%
by weight of BaCrO4 core particles capsulated with a SiO2 membrane and 1% of weight of additional TiO2 colloidal particles, the first electroplating liquid had the following composition.
ZnSO4 7H2O 180 g/l CoSO4 7H2O 10 to 450 g/l BaCrO4 core particle encap-sulated by SiO2 membrane 5 to 60 g/l TiO2 0.5 to 60 g/l In each of Example 2, 6 to 12, 16 to 19, 23, 27, 28, 30 to 32, 35, 37 and 38, an additional electro-plating layer in the total amount of 1 to 5 g/m2 and the composition a~ shown in Table 1 was formed on the base electroplating layer surface by using a second electroplating liquid containing necessary metal ions, for example, Zn ions or a mixture of Zn ions with Fe, Co, Ni, Mn and/or Cr ions in the form of sulfates.
In each of Examples 3, 4, 6, 8, 10, 13 to 15, 20, 21, 24, 25, 28 to 30, 32, and 35 to 38, a surface coating layer having the composition and the thickness as shown in Table 1 was formed on the base electro-plating layer or the additional electroplating layer.
In the formation of the surface coating layer, the organic resinous material layer or chromium-containing organic resinous material layer was formed by a roll-coating method and by using a water-soluble polyacrylic ~ - 27 ~ 1 334 018 resin emulsion. Also, the chromate treatment was carried out by coating, reaction or electrolysis.
The resultant plated composite steel strip was subjected to the following tests.
1. Cyclic corrosion resistance test A painted specimen, which was prepared by a full-dip type chemical conversion treatment and a cationic paint-coating, and an unpainted specimen, were scratched and then subjected to a 50 cycle corrosion test. In each cycle of the corrosion test, the specimens were subjected to salt water-spraying at 35C
for 6 hours, to drying at 70C at 60%RH for 4 hours, to wetting at 49C and at a 95%RH or more for 4 hours, and then to freezing at -20C for 4 hours.
After the 50 cycle corrosion test, the formation of red rust and the depths of pits formed in the specimens were measured.
2. Paint adhesion property A specimen was subjected to a full-dip type chemical conversion treatment, was coated three times with paint, and was then immersed in hot water at 40C
for 10 days.
After the completion of the immersion step, the specimen was subjected to a cross-cut test in which the specimen surface was scratched in a chequered pattern at intervals of 2 mm to form 100 squares. Then an adhesive tape was adhered on the scratched surface of the specimen and was peeled from the specimen. The number of squares separated from the specimen was then counted.
The rust resistance was evaluated as follows.
Class Rust formation R (%) R = 0 4 R < 5 3 5 < R < 20 2 20 < R < 50 1 50 < R

- - 28 - 133~018 The depth of corrosion was evaluated as follow~.
Class De~th C (mm) of DitS
C = 0 4 C < 0.1 3 0.1 < C ~ 0.3 2 0.3 < D < 0.5 1 0.5 < C
The paint-adhesion property was evaluated as follows.
Class Peeled sauares D (%) D = 0 4 D < 5 3 5 < D < 20 2 20 < D < 50 1 50 < D
The results of the tests are shown in Table 1.

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l~z C~ , _ 35 _ 1334018 Table 1 clearly shows that the plated composite steel strips of Examples 1 to 38 in accordance with the present invention exhibited an enhanced corrosion resistance and a satisfactory paint-adhesion in com-parison with the comparative plated composite steelstrip. Namely, the specific corrosion-preventing fine particles in the form of microcapsules are effective for promoting the corrosion resistance of the resultant plated composite steel strip.

Claims

1. A high corrosion resistant electroplated composite steel strip comprising:

(A) a substrate consisting essentially of a steel strip;
and (B) at least one corrosion resistant coating layer formed on at least one surface of the steel strip substrate and comprising at least a base plating layer which comprises (a) a matrix consisting of a member selected from the group consisting of zinc and zinc alloys, and (b) a plurality of corrosion-preventing fine solid particles in the form of microcapsules dispersed in the matrix and consisting essentially of (i) fine core solid particles comprising at least one member selected from the group consisting of chromates, aluminum compounds, phosphates, molybdenum compounds and titanium compounds; and (ii) membranes encapsulating the fine core solid particles therein and comprising at least one member selected from the group consisting of SiO2, TiO2, Al2O3, ZrO2, ethyl cellulose, amino resins, polyvinylidene chloride resins, polyethylene resins and polystryrene resins.

2. The composite steel strip as claimed in claim 1, wherein the membranes have a thickness of 1.0µm or less.

3. The composite steel strip as claimed in claim 1, wherein the base plating layer is present in an amount of 6 to 50 g/m2.

4. The composite steel strip as claimed in claim 1, wherein the corrosion-preventing particles are present in a total amount of 0.1% to 30% by weight based on the weight of the base plating layer.

5. The composite steel strip as claimed in claim 1, wherein the corrosion resistant coating layer has a surface coating layer formed on the base plating layer, said surface coating layer comprising a single layer structure comprising a member selected from organic resinous materials and mixtures of at least one of the organic resinous materials and chromium ions.

6. The composite steel strip as claimed in claim 1, wherein the corrosion resistant coating layer has a surface coating layer formed on the base plating layer, said surface coating layer comprising a double layer structure consisting essentially of an under layer formed by applying a chromate treatment to the base plating layer surface and an upper layer formed on the under layer and comprising an organic resinous material.

7. The composite steel strip as claimed in claim 1, wherein the zinc alloy consists of Zn and at least one additional metal member selected from the group consisting of Fe, Co, Mn, Cr, Sn, Sb, Pb, Ni and Mo.

8. The composite steel strip as claimed in claim 1, wherein the base plating layer further contains a number of additional fine or colloidal particles comprising at least one member selected from the group consisting of SiO2, TiO2, Cr2O3, Al2O3, ZrO2, SnO2, and Sb2O5.

9. The composite steel strip as claimed in claim 8, wherein the additional particles are present in an amount of 0.1 to 30% by weight based on the weight of the base plating layer.

10. The composite steel strip as claimed in claim 1, wherein the corrosion resistant coating layer has an additional thin electroplating layer formed on the base plating layer, the additional thin electroplating layer comprising at least one member selected from the group consisting of Zn, Fe, Co, Ni, Mn and Cr.

11. The composite steel strip as claimed in claim 10, wherein the additional thin electroplating layer is present in an amount of 1 to 5 g/m2.

12. The composite steel strip as claimed in claim 10, wherein the corrosion resistant coating layer has a surface coating layer formed on the additional thin electroplating layer, said surface coating layer comprising a single layer structure comprising a member selected from organic resinous materials and mixtures of at least one of the organic resinous materials and chromium ions.

13. The composite steel strip as claimed in claim 10, wherein the corrosion resistant coating layer has a surface coating layer formed on the additional thin electroplating layer, said surface coating layer comprising a double layer structure consisting essentially of an under layer formed by applying a chromate treatment to the base plating layer surface and an upper layer formed on the under layer and comprising an organic resinous material.

14. A method of producing a high corrosion resistant electroplated composite steel strip comprising coating at least one surface of a substrate consisting essentially of a descaled steel strip by at least first electroplating at least one surface of the substrate with a first electroplating liquid containing (a) matrix-forming metal ions selected from the group consisting of zinc ions and mixtures of ions of zinc and at least one metal other than zinc to be alloyed with zinc, (b) a plurality of corrosion-preventing fine solid particles dispersed in the electroplating liquid and consisting essentially of fine core particles encapsulated by organic or inorganic coating membranes, and (c) a co-deposition-promoting agent for promoting the co-deposition of the corrosion-preventing fine particles together with the matrix-forming metal, to form a base plating layer on the substrate surface, said fine core particles comprising a member selected from the group consisting of chromates, aluminum compounds, phosphates, molybdenum compounds and titanium compounds, which are all soluble in the first electroplating liquid;
said coating membranes comprising at least one member selected from the group consisting of SiO2, TiO2, Al3O2, ZrO2, ethyl cellulose resin, amino resins, polyvinylidene chloride resins, polyethylene resins and polystryrene resins which are all substantially insoluble in the first electroplating liquid; and said co-deposition-promoting agent comprising at least one member selected from the group consisting of Ni2+ ions, Fe2+ ions, Co2+ ions, Cr3+ ions, TiO2 colloid, Al2O3 colloid, SiO2 colloid, ZrO2 colloid, SnO2 colloid and Sb2O5 colloid and, amine compounds having a cationic polar structure of the formula (1):

(1) ammonium compounds having a cationic polar radical of the formula (2):

(2) in which in formulae (1) and (2), R1, R2, R3 and R4 represent, respectively and independently from each other, a member selected from the group consisting of a hydrogen atom, an alkyl radical and an aryl radical, and polymers having at least one member selected from the group consisting of the cationic polar radicals of the formulae (1) and (2).

15. The method as claimed in claim 14, wherein the corrosion-preventing fine particles contain chromium, a portion of the chromium is dissolved into the first electroplating liquid to form Cr6+ ions in the first liquid and the Cr6+ ions are reduced into Cr3+ ions by adding metal grains, a metal plate or a reducing agent in a necessary amount for reducing the dissolved Cr6+
ions into Cr3+ ions in the first liquid.

16. The method as claimed in claim 14, wherein the first electroplating liquid contains zinc sulfate and has a pH of 3.5 or less.

17. The method as claimed in claim 16, wherein the first electroplating step is carried out in the first electroplating liquid containing zinc sulfate by using an insoluble electrode.

18. The method as claimed in claim 14, wherein the first electroplating liquid contains additional fine or colloidal particles comprising at least one member selected from the group consisting of SiO2, TiO2, Cr2O3, Al2O3, ZrO2, SnO2 and Sb2O5.

19. The method as claimed in claim 14, wherein the first electroplating step is followed by a second electroplating step comprising electroplating the base plating layer with a second electroplating liquid containing at least one member selected from the group consisting of Zn, Fe, Co, Ni, Mn and Cr ions, to form an additional thin electroplating layer.

20. The method of claim 19, wherein the second electroplating step is followed by surface coating the additional thin electroplating layer in a manner such that an organic resinous material optionally containing chromium ions evenly mixed therein is coated on the additional thin electroplating layer surface to form a single coating layer, or such that an under layer is formed by applying a chromate treatment to the additional thin electroplating layer surface and then an upper layer comprising an organic resinous material is formed on the under layer surface to form a double coating layer structure.

21. The method as claimed in claim 14, wherein the first electroplating step is followed by surface coating the base plating layer in a manner such that an organic resinous material optionally containing chromium ions evenly mixed therein is coated on the base plating layer surface to form a single coating layer, or such that an under layer is formed by applying a chromate treatment to the base plating layer surface and then an upper layer comprising an organic resinous material is formed on the under layer surface, to form a double coating layer structure.